1. Field of the Invention
This invention concerns a short-arc type discharge lamp used for a light source in such applications as projectors, exposure or semiconductors, ultraviolet hardening and so on.
2. Description of Related Art
Short-arc discharge lamps have been used, for example, as light sources in light source equipment, such as projectors or other light projection equipment. In such short-arc discharge lamps, a cathode and anode have faced each other within a bulb made of quartz glass, in which xenon gas, for example, was sealed.
FIG. 5 is a cross-sectional explanatory drawing that shows one example of a conventional short-arc discharge lamp. The bulb 30 of this short-arc discharge lamp comprises a light-emitting tube 31 to which an outwardly extending sealing tube 32 is connected at each side. A constricted portion 32a, which is a part of the sealing tube 32, is formed where the sealing tube 32 joins the light-emitting tube 31.
Within the light-emitting tube 31, a cathode 33 and anode 34 are located facing each other. The cathode 33 and the anode 34 are each mounted on the end of an electrode rod which passes through a sleeve material 35 of quartz glass, for example, and projects outward through the sealing tube 32. An air seal 37 is formed by fusing the end of the sealing tube 32 to the electrode rod 35. The sleeve material 36 is fastened in place by fusing it into a single piece with the constricted portion 32a. In this way, the electrode rod 35 is held in place by the sleeve material 36 and the end of the sealing tube 32.
FIG. 6 is a cross-sectional explanatory drawing of a key part of the short-arc discharge lamp shown in FIG. 5. A pendant ridge 38 formed when the constricted portion 32a of the sealing tube 32 is formed extends around the full inner circumference of the sealing tube 32 near to the end of the sleeve material 36 which is closest to the light-emitting tube 31. A wedge-shaped space 39 is thus formed between the pendant ridge 38 and the end of the sleeve material 36 that is closest to the light-emitting tube 31. The constricted portion 32a of the sealing tube 32 is formed by reducing the diameter of the sealing tube 32, by means of heating a portion of the outer periphery of the sealing tube 32 while the pressure within the bulb is reduced, for example, to between 1xc3x9710-3 and 1xc3x9710-4 Torr.
However, in recent years, there have been demands for greater lighting power in these short-arc discharge lamps. To satisfy these demands, it has become necessary to increase the amount of gas sealed into the light-emitting tube 31, leading to the problem that, when the amount of gas sealed into the light-emitting tube 31 of this kind of short-arc discharge lamp is increased, the pressure of the gas sealed within the light-emitting bulb 31 increases to perhaps 7500 Torr or more when the lamp is lit. The sleeve material 36 and the constricted portion 32a are fused into a single piece, so that the stress is concentrated on the wedge-shaped space 39 and cracks appear in the constricted portion 32a. When the lamp is turned on and off repeatedly, the cracking progresses until finally the sealing tube 32 is broken.
In order to prevent cracks from appearing in the sealing tube 32 in this way, it has been proposed that the sleeve material 36 be fixed in place while preventing the constricted portion 32a of the sealing tube 32 and the sleeve material 36 from fusing into a single piece, by means of placing molybdenum foil between the sleeve material 36 and the constricted portion 32a. It has been found, however, that even with such a construction, it is not possible to completely resolve the problem described above, and cracks sometimes appear in the constricted portion when the lamp is turned on and off repeatedly.
This invention was perfected as a result of long research to understand the causes of the problem stated above. Its purpose is to reliably prevent the appearance of cracks in the sealing tube, and thus to provide a short-arc discharge lamp with a long service life.
The short-arc discharge lamp of this invention is one having a bulb comprising a light-emitting tube and sealing tubes connected to both sides of the light-emitting tube, with an anode and a cathode positioned facing each other within the light-emitting tube, the anode and cathode being on the ends of electrode rods that pass through sleeve material that is supported by the sealing tubes, in which short-arc discharge lamp, molybdenum foil is located between the sleeve material and the sealing tube, the outer surface of the molybdenum foil which faces the sealing tube having undergone surface treatment with a high-melting-point metal that is resistant to oxidation. The high-melting-point metal is preferably chosen from among tungsten, rhenium, tantalum, rhodium and platinum.
In the arrangement described above, molybdenum foil is located between the sleeve material and the sealing tube, and the outer surface of the molybdenum foil which faces the sealing tube has undergone surface treatment with a high-melting-point metal with resistance to oxidation. Because of this, the molybdenum foil does not fuse with the sealing tube, and it is possible to prevent the appearance of cracks in the sealing tube, and as a result, it is possible to obtain a short-arc discharge lamp with long service life.
These and further objects, features and advantages of the present invention will become apparent from the following description when taken in connection with the accompanying drawings which, for purposes of illustration only, show a single embodiment in accordance with the present invention.